Macromolecules 1987,20, 38-44
38
gratefully acknowledged. Thanks are also due to S. K. Roy and A. Majundar, who performed the dynamic mechanical properties measurements, and to Dr. Y. Nishio for helpful discussions. Registry No. HPC,9004-64-2.
(12) (13) . .
References and Notes
(14) (15)
(1) Gray, D. G. J . Appl. Polym. Sci., Appl. Polym. Symp. 1983, 37, 179. (2) Gilbert, R. D.; Patton, P. A. Prog. Polym. Sci. 1983, 9, 115. (3) Gray, D. G . Faraday Discuss. Chem. SOC. 1985, 79, 257. (4) Werbowyj, R. S.; Gray, D. G. Macromolecules 1980, 13, 69. (5) Werbowyj, R. S.; Gray, D. G. Macromolecules 1984,17,1512. (6) Shimamura,K.; White, J. L.; Fellers, J. F. J. Appl. Polym. Sci. 1981,26, 2165. (7) Suto, S.; White, J. L.; Fellers, J. F. Rheol. Acta 1982, 21, 62. (8) Bhadani, S. N.; Gray, D. G. Mol. Cryst. Liq. Cryst. 1984,102, 255. (9) Shannon, P. Macromolecules 1984, 17, 1873.
(10) (11)
(16) (17) (18) (19) (20) (21) (22)
Finkelmann, H.; Rehage, G . Makromol. Chem., Rapid Com-
mun. 1980, 1, 733. Nishio, Y.; Yamane, T.; Takahashi, T. J . Polym. Sci., Polym. Phys. Ed. 1985,23, 1043. Tsutsui, T.; Tanaka, R. Polymer 1980, 21, 1351. Krinbaum. W. R.: Cifferi.. A.:, Asrar.. J.:. Toriumi. H. Mol. Crvst. Liq: Cryst. 1981,' 76, 79. Chiellini, E.; Galli, G. Macromolecules 1985, 18, 1652. Ito, K.; Kajiyama, T.; Takayanagi,M. Polym. J . (Tokyo)1977, 9, 355. (a) Watanabe, J.; Sasaki, S.; Uematsu, I. Polym. J. (Tokyo) 1977,9, 451. (b) Watanabe, J.; Naka, M.; Watanabe, J.; Watanabe, K.; Uematsu, I. Polym. J . (Tokyo) 1978,10, 569. Samuels, R. J. J . Polym. Sci., Part A-2 1969, 7, 1197. Navard, P.;Haudin, J. M. Polym. Sci. Technol. 1985,28,389. de Vries, H. Acta Crystallogr. 1951, 4 , 219. Bhadani, S. N.; Gray, D. G. Mol. Cryst. Liq. Cryst. 1983,99, 29. Aspler, J. S.; Gray, D. G. Polymer 1982, 23, 43. Suto, S.; Kudo, M.; Karasawa,M. J. Appl. Polym. Sci. 1986, 31, 1327.
Synthesis and Characterization of Pyrene-Labeled (Hydroxypropy1)celluloseand Its Fluorescence in Solution Frangoise M. Winnik,*' Mitchell A. Winnik,2 and Shigeo Tazuke Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Japan Christopher K. Ober Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2L1. Received July 21, 1986 ABSTRACT: The fluorescence spectroscopy of pyrene-labeled (hydroxypropy1)cellulose was examined in solution in various alcohols and in water. The low relative excimer fluorescence intensity in alcohol solvents such as methanol is typical of a randomly coiled polymer containing a small fraction of pyrene groups. The very intense excimer intensity in water suggesta that the polymer adopts a different conformation in aqueous solution, characterized by extensive ground-statedimerization and aggregation of the pyrene groups. There is also strong evidence for polymer-polymer association, even at very low polymer concentration, which also contributesto the extent of excimer emission. Other experiments involving fluorescence and UV absorption measurements on these samples also support this view.
(Hydroxypropy1)cellulose(HPC) is a polymer with many important industrial applications. Like (hydroxymethy1)cellulose and (hydroxyethyl)cellulose, HPC is a soluble derivative obtained by chemical functionalization of cellulose. Scientific interest in these polymers derives from the fact that they are semiflexible, nonionic, linear polymers3 and that they form liquid crystalline states in the melt and in concentrated solutions.4 A variety of their properties have been investigated, including dilute solution viscosity,3p5 fluorescence depolarization in sheared solutions? and polymer interaction with surfactants in aqueous solutions.6b Fluorescence labeling techniques have proved useful in studying a wide range of properties of various polymers and their mixtures. We anticipate that these techniques will also be useful in the study of HPC-containing systems. In this paper we describe the synthesis of pyrene-labeled HPC (HPC-Py), its characterization, and some of its fluorescence properties in dilute solution. In order to carry out experiments in aqueous solution without concern for the possible loss of label through hydrolysis, we chose to attach the pyrene groups by means of an ether linkage. A convenient method involves treating *To whom correspondence should be addressed. 0024-9297/87/2220-0038$01.50/0
a solution of HPC with a strong base to generate alkoxide sites, followed by reaction with 4 41-pyreny1)butyl tosylate. The pyrenes become attached, presumably, to oxypropyl pendant groups, leading to the kind of structure depicted below: OH
OH
I
OH
OCH2CHCH3
I
I W
H
0Cn2CHcH3 I
on
OCH2CHCWz
I
OCH2CHCH3
I
CH2
n
yn2
OCH~CHCH~
I
OCH2CHCH3
I
on
OCH2CHCH3
From our experiments, we show, for example, that the pyrene groups are attached to sites of considerable local flexibility. In alcohol solvents, the pyrene groups are predominantly unassociated, but form excimers by a dynamic mechanism after photoexcitation. In water, a different behavior is observed. The pyrenes stack or dimerize in a face-to-face configuration, so that the excimer and monomer emissions derive from different populations of 0 1987 American Chemical Society
Macromolecules, Vol. 20, No. 1, 1987
pyrene groups. Various cosolvents or cosolutes can interact with HPC-Py to affect the extent of pyrene association in the ground state.
Experimental Section Materials. (Hydroxypropy1)cellulose(HPC, Klucel L., Hercules Inc.) was purchased from Aldrich Chemical Co. The manufacturer's literature claims a molecular weight of 100000. Other groups have reported measurements consistent with this value. One recent reference reports M(sedimentati0n) = 82000;" and another, 73 OOO with M , = 36 OO0.8b Spectral grade solvents were used. The distilled, deionized water had a conductance less than 5 X lo-' i2-l cm-'. 4-(l-Pyrenyl)butanolewas prepared by reduction of 4-(l-pyrenyl)butyric acid with lithium aluminum hydride in tetrahydrofuran (THF) and was recrystallized from ethyl acetate/hexane (21 (v/v)). Dimethylformamide (DMF) was dried by reflux over calcium hydride, followed by distillation a t reduced pressure. Synthesis. 4-(1-Pyreny1)butylTosylate (2).'O T o a solution of 4-(l-pyrenyl)butanol(l.Og, 3.65 mmol) in chloroform (10 mL, flushed through alumina) were added first pyridine (0.58 g, 7.3 mmol) and then p-toluenesulfonyl chloride (1.04 g, 5.48 mmol) in small portions over a period of 5 min. The mixture was stirred at ca. 22 "C in the dark under nitrogen for 1.5 h. The reaction mixture, diluted to 20 mL with diethyl ether, was extracted successively with 10% aqueous HCl (twice), water (twice), 5% aqueous sodium bicarbonate solution (twice), water (once), and saturated brine (twice). The organic layer was dried over MgS04 and evaporated to yield an oil. Crystallization from ethyl acetate/hexane (25 mL, 1:2 (v/v)) yielded 2 (1.34 g, 86%): mp 91-92 OC; A- (e, THF) 343 nm (38500), 327 (25000); 'H NMR (CDCI,, 6) 1.6-2.0 (m, 4 H), 2.22 (s, 3 H), 3.27 (br t, 2 H), 4.06 (br t, 2 H), 7.1-8.3 (m, 13 H). Pyrene-Labeled(Hydroxypropy1)cellulose(HPC-Py). To prepare the sample of higher pyrene content (HPC-Py/ 26), a solution of 4-(l-pyrenyl)butyl tosylate (250 mg, 5.8 mmol) in dry DMF (1.0 mL) was added to a solution of HPC (2.0 g) in dry DMF (15 mL). The HPC had previously been dried by azeotropic distillation of water from solutions of the polymer in toluene (three times). The mixture was stirred at ca.22 "C for 2 h. Then sodium hydride (100 mg, 60% dispersion in oil, washed twice with dry hexanes) in DMF (0.5 mL) was added, and the mixture was allowed to stand overnight. Excess base was neutralized with diluted acetic acid (3 mL, 3:l (v/v) with water). Solvents were removed a t 55 "C at reduced pressure. The residual material was dissolved in T H F (20 mL). Addition of hexanes led to the precipitation of the polymer, which was separated by decanting off the supernate. This reprecipitation was repeated three times. The final product was dried in vacuo a t 65 "C for 24 h to yield 0.95 g of a white amorphous solid. GPC analysis indicated that all Py groups in the sample were attached to the polymer chain. By UV analysis, using 4-(l-pyrenyl)butanol as a model, the pyrene content was calculated to be 1.23 X mol/g polymer. This corresponds to 1pyrene per 26 glucose units in a polymer of M , = 36 000. The denominator in HPC-Py/26 represents the average number of glucose units per pyrene group in the polymer. A sample of lower Py content (HPC-Py/216) was prepared similarly, using 2.0 g of HPC, 30 mg of 4-(l-pyrenyl)butyl tosylate (0.07 mmol), and NaH (100 mg). The pyrene content in the final polymer (1.9 g) was calculated to be 1.37 X lod mol/g, equivalent to 1 Py per 216 glucose units for a polymer of M, = 36000. Instrumentation. lH NMR spectra were recorded at 80 MHz with a Bruker WP-80 spectrometer. Spectra were run in chloroform-d containing 0.1% Me,Si as an internal standard. UVvisible spectra were recorded either with a Hewlett-Packard 845OA diode array spectrometer or with a Shimadzu Model 200s spectrometer. Steady-state fluorescence spectra were run on a Hitachi Model 4000 fluorescence spectrometer. Fluorescence decay measurements were made with a time-correlated single-photoncounting instrument purchased from Photochemical Research Associates (London, Ontario, Canada). Molecular weights and molecular weight distributions were estimated by gel permeation chromatography (GPC), using a Hewlett-Packard 1090 instrument equipped with a 1037A refractive index detector and a 8194A diode array UV-visible detector set at 278,316, 328, and 344 nm. The
Pyrene-Labeled (Hydroxypropy1)cellulose 39 columns used were PL-gel IO6, lo4, lo3,500, and 300 A. Spectral grade T H F (BDH) was used as the solvent with a flow rate of 1.0 mL/min. Fluorescence Measurements. Spectra were fully corrected. The excitation slits were set at 5 nm. Emission slits were normally set at 5 nm, although some experiments were run with 3-nm slits. Because the excitation spectra of the pyrene monomer and excimer emission were different for the polymer samples in aqueous solution, the excitation wavelength had to be selected carefully. A wavelength of 330 nm was chosen for most experiments, since it is 2 nm to lower wavelength of a maximum in the excimer excitation spectrum and 2 nm to longer wavelength from the corresponding maximum in the monomer spectrum. For solutions in alcohol solvents and for quantum yield measurements in mixed solvents, the excitation wavelength was the peak maximum in the W spectrum closest to 330 nm. All measurements were made a t 20 "C. Samples were degassed by vigorous bubbling of solvent-saturated argon for 30 s immediately prior to the measurement. It was found that continued bubbling for longer periods of time made no difference, whereas insufficient degassing, particularly for methanol solutions, led to a reduced fluorescence intensity and in some cases to changes in the excimer/monomer In aqueous solution, emission fluorescenceintensity ratio (ZE/Z~). intensities were unaffected by degassing. Most of these experiments were carried out on air-saturated solutions. Peak heights and peak areas were proportional. Quantum yields were calculated by integration of peak areas after conversion of the spectra of wavenumber units, using quinine sulfate as a standard. Beer's law corrections were applied as necessary for optical density changes, whereas index of refraction corrections were negligible. In aqueous solutions, these values are apparent quantum yields, because monomer and excimer emission derive from different species that absorb light at very similar wavelengths. Samples for Spectroscopic Analysis. HPC-Py solutions were prepared a t room temperature (19 "C) by allowing the polymer to swell and then dissolve in excess solvent. Dissolution in methanol, ethanol, and ethoxyethanol was rapid. In water, glycerol, and diethylene glycol, dissolution was slow. The mixtures were allowed to stand typically 24 h before they were diluted to a known total volume. For the HPC-Py/26 sample, the solutions for spectroscopic analysis contained approximately 5 mg of polymer in 100 mL of solvent. For the sample of lower Py content, 40 mg in 100 mL was used.
Results Synthesis and Characterization of HPC-Py.Linkage of the pyrene chromophore to HPC was achieved by reaction of the sodium alkoxide derivative of HPC with 4-(l-pyrenyl)butyl tosylate, prepared from the corresponding alcohol. Because of the tendency of HPC alkoxides to form thick gels in DMF, it was necessary to invert the normal order of adding reagents in the Williamson ether synthesis. Instead of adding the tosylate to the preformed alkoxide, the tosylate was mixed first with a solution of carefully dried HPC in DMF. When this solution became clear, indicating that all the tosylate had dissolved, then the sodium hydride was added. Under these circumstances, the diffusion distance of the tosylate to the reactive sites on the polymer was small, and the reaction proceeded in high yield. The extent of pyrene incorporation into the polymer was controlled through the ratio of the reactants. The purity of the HPC-Py was an important aspect of these experiments. Through the use of tandem UV-visible and refractive index detectors for the GPC analysis, it was established that the pyrene groups were covalently linked to the polymer, that after several reprecipitations the sample contained less than 0.1% low molecular weight pyrene impurities, and that the chemical transformation did not alter the molecular weight or the (broad) molecular weight distribution of the polymer. The concentration of pyrene chromophores incorporated into the polymer was determined by UV absorption
Macromolecules, Vol. 20, No. I , 1987
40 Winnik et al. Table I Peak-to-Valley Ratios for Pyrene Absorption and Excitation Spectra OD(343)/ I(343) /I(334) sample OD(334) A,, = 379 A, = 489 ZE/IM In Water Py(CHJ40H 2.0 2.45 1.75 0.10 HPC-Py/216 2.16 2.34 1.89 2.12 1.75 2.01 HPC-Py/26’ p~(CHd40H HPC-Py/216 HPC-Py/26
2.42 3.50 3.20
In Methanol 2.82 2.80 2.71
2.44
Table I1 Values of the Pyrene Excimer-to-MonomerFluorescence Intensities for HPC-Py/216 and HPC-Py/26 in Water and Alcohols solvent water methanol ethanol ethoxyethanol diethylene glycol glycerol
0.04 0.31
0.13
1.60 0.31 0.31 0.23
0.05
0.05 c0.02
